1. Field of the Invention
The present invention relates to a device for determining a misfiring cylinder of a multicylinder engine.
2. Description of the Related Art
When one of the cylinders in a multicylinder engine misfires, the engine rotational speed drops at the power stroke of the misfiring period and therefore the time required for the crankshaft to turn by a certain crank angle during the power stroke of the misfiring cylinder becomes longer than that of other cylinders.
Therefore, for example, there is known a multicylinder engine where it is judged, for example, that the no. 1 cylinder has misfired when the period required for the crankshaft to turn by a certain crank angle in the power stroke of the no. 1 cylinder becomes longer than that of other cylinders (see Japanese Unexamined Patent Publication (Kokai) No. 62-228929).
In such a multicylinder engine, however, there are provided a rotor which is made to rotate synchronously with the crankshaft and which is formed with outer teeth and an electromagnetic pickup which is arranged in proximity to the outer teeth of the rotor and which produces an output pulse when facing an outer tooth. The time required for the crankshaft to turn by a certain crank angle is found from the time interval of generation of these output pulses. The outer teeth of the rotor, however, suffer from large manufacturing error and therefore when trying to find the time required for the crankshaft to turn by a certain crank angle from the time interval of generation of the output pulses in the above way, there will be a difference in the time required for the crankshaft to turn by a certain crank angle in the power stroke of the no. 1 cylinder, for example, and the time required for the crankshaft to turn by a certain crank angle in the power stroke of the other cylinders even when no misfire occurs.
Even if such a difference occurs, a misfiring cylinder usually causes the rotational speed to fall considerably, so there would be no mistaken judgement if judging that a misfire has occurred when the difference is more than a relatively large set value. During high speed engine operation, however, the power stroke becomes shorter in duration, so even if a misfire occurs, the rotational speed does not drop that much. Also, during low load engine operation, the output torque is inherently low, so the rotational speed will not drop that much even if a misfire occurs. Therefore, when judging that a misfire has occurred when the difference is greater than a relatively large set value as mentioned above, it is not possible to detect the occurrence of a misfire during high speed engine operation and low load engine operation. On the other hand, if the setting is made lower so as to enable detection of a misfire during high speed engine operation and low load engine operation, the result will be a mistaken judgement of a misfire occurring even when it has not and therefore the setting has to be made a relatively large value. In the final analysis, there has been the problem that it was not possible to detect a misfire at operating regions such as during high speed engine operation and low load engine operation where the engine rotational speed does not fall that much even when a misfire occurs.